The present application relates to imaging systems, and in particular to imaging systems which utilize arrays of reflective elements, and high intensity power light sources. Each of the reflective elements having two driven states which are electrostatically activated against a flexural restoring force. The array of reflective elements can be implemented in a number of different configurations. Among the possible configurations is an array arrangement that uses hinged mirrors which are titled or rotated by an appropriate force. Another implementation is the use of a suspended reflective film which can be pulled electrostatically towards a transparent ITO electrode on a transparent window, or pulled in the opposite direction towards another electrode. In each case stress would induce an undesirable flexural relaxation in the stressed direction which results in a deleterious effect on the overall array operation.
While the above array implementations have been specifically mentioned, and the following discussion primarily focusses on switching mirror arrays, it is to be understood the present concepts are applicable for any array type configuration where deleterious effects are introduced to the array arrangement due to array switching operations while in an system using a high intensity power light source.
In existing systems mirrors of a switchable mirror array have associated hinge arrangements which allow the individual mirrors to be tilted or rotated in a manner that reflects light incident on the mirror surface. Rotations of the individual mirrors operate in accordance with image data from an electronic controller, which controls operation of the mirror array by applying voltages to one corner (or side) of the mirror or its opposite corner (or side).
When in a mirror OFF state, individual mirrors will reflect the incident light away from a downstream processing section of the imaging system, and when in a mirror ON state, individual mirrors will reflect the incident light to the downstream processing section of the imaging system.
While not limited to, in certain embodiments, the mirror array may be a torsional micro-mirror array configuration, where hinges of the array are composed of amorphous materials including but not limited to aluminum. Further, the high intensity power light source in certain designs is a laser or laser configuration.
It is known that in imaging systems which employ a high intensity power light source, the light energy impinging on the mirror array will cause heating which in turn may result in deleterious plastic deformation of the hinges associated with the individual mirrors of the mirror array. This deformation will often result in hinge creep, which essentially causes individual mirrors of the mirror array to become offset from a pre-determined desired angle. Above a certain level of hinge deformation a mirror may no longer be switchable at the operational switching voltage.
This situation is most commonly found in imaging systems which consist of image patterns having something other than overall 50% mirror ON-times and 50% mirror OFF-times. As more mirrors exhibit the hinge creep effect at higher temperatures, reliability and life expectancy of the entire mirror array is diminished.
However, it is known that by balancing a mirror in opposite stress states, the lifetime of mirror arrangements can be enhanced. For example if a torsional mirror hinge is twisted 50% of the time towards an ON state rotation and 50% of the time towards an OFF state rotation, stress bias is greatly suppressed as creep in one direction reverses creep in the other.
An illustrative example of hinge creep is shown in FIGS. 1A-1C. Particularly, mirror assembly 100 includes a mirror 102, associated hinges 104 and 106, and a controller 108. It is understood the hinges, mirror and controller in these figures are simplified illustrations and commercial versions of these arrangements are more detailed. It is further understood the present discussion is relevant to the mirror arrangements that employ a variety of hinge configurations beyond that shown here. Also, in at least one implementation the switching or rotating is accomplished by electrostatically pulling the mirror in a desired direction.
In FIG. 1A, mirror 102 is shown rotated to a +12° angle when in an ON state, and when mirror 102 is moved to an OFF state it will be at −12°, as shown in FIG. 1B. The rotation of the mirror is accomplished by operation of hinges 104 and 106 under control of controller 108, as is well-known in the art. It is noted that each rotation of the mirror passes through 0°, which is the intended position of the mirror 102 when the mirror array has been powered down.
As shown in FIG. 1C, due to heating caused by a high intensity power light source, deleterious plastic deformation of hinges 104 and 106 has occurred. Particularly, due to the impingement of high intensity light, the whole mirror array structure is heated, allowing the thermally activated processes of dislocation generation and migration to be more probable. Then through dislocation motion in the hinge material (e.g., aluminum) of the mirror array, the hinge stress in the ON or OFF tilt state tends to be relaxed and the hinge takes on a ‘set’. The hinge no longer has its original shape, straight for example, but deviates from its original shape (has a curvature in the present example) such that the mirror is tilted towards the stressed state when power is removed. Also, the stress needed to be overcome in switching to the opposite state is now larger. Then, when power to the mirror array is turned off, instead of mirror 102 lying flat at 0°, the lattice of the mirror arrangement has been deformed, and the mirror is not able to return to 0° (i.e., in the example of FIG. 1C only being able to rotate back to a −5° angle), which means when the mirror array is powered back up and attempts are made to switch between the desired +12° (ON) and −12° (OFF), rotation to the +12° (ON) state cannot be achieved. It is to be appreciated that the angles discussed herein are exemplary, and particular imaging systems may have different desired angle positions and flexural configurations. For clarity, it is further explained that the “OFF” state is a distinct concept from power being turned off to the mirrors. When the power supply is off no voltage is applied to either the ON or OFF electrodes, and the mirrors return to their powered down state (e.g., 0° for a well working array, or for the damaged array −5°)
As mentioned above, in imaging systems where operation results in an overall set of image patterns requiring substantially 50% mirror ON times and 50% mirror OFF times, the associated tilt and/or rotation operations act to offset the effects of the heating of the hinges and plastic deformation, and will allow the individual mirrors to be switchable to the expected angles. It is worth mentioning here that for certain array devices, the mirrors are set at the ON and OFF positions by mechanical stops. Other arrays could be more analog in operation, in which case the creep annealing would be even more of an issue. For the ON and OFF type devices, it is only necessary that the mirror be able to be pulled to the state opposite from that of the deformed hinge. For the analog case, the mirror position is more a balance between the electrostatic force and the hinge flexure counter force. In this case the angle of the mirror would also depend on residual deformation of the hinge.
In understanding the above, the existing art has shown that even in systems where image creep is not perfectly balanced during operation, actions can be taken to suppress its effect by exercising the mirrors after all imaging operations have been completed, but prior to turning off (or immediately after turning off) the high intensity power light source (i.e., where exercising of the mirrors includes rapidly switching between ON and OFF states of the individual mirrors). The exercising of the mirrors therefore acts to unwind hinge creep while the imaging system is still in a high temperature state.
The present application provides systems and methods to improve the avoidance and/or reduction of hinge creep in imaging systems which utilize switching mirror array technology and high intensity power light sources.